In his address in Saudi Arabia on Sunday, May 21, while calling on Muslim leaders to lead the fight against terrorism, President Donald Trump identified Iran as a despotic state giving safe harbor and financing terror in the Middle East. As Iran is a Shia state and Saudi Arabia a Sunni-led country, some media outlets criticized Trump for taking sides in the Shia-Sunni sectarian divide.

As a scholar of Islam and a public educator, I often field questions about Sunnis, Shias and the sects of Islam. What exactly is the Shia-Sunni divide? And what is its history?